1. Field of the Invention
The subject invention relates to resonant mode power supplies, e.g., for television receivers.
2. Description of the Related Art
Television receivers include power supplies for supplying power to the various circuit components. Generally, these power supplies are in the form of switched-mode power supplies in which a dc voltage is applied to one end of the primary winding of a flyback transformer. The other end of the primary winding is connected to ground through a switching element which is controlled to switch on and off at a predetermined frequency. The switched power to the transformer is then transferred to the secondary winding for providing output power.
It has been found that resonant mode power supplies have higher efficiencies and are especially suited for power levels higher than 150 W and for power supplies that need to be miniaturized, e.g., power supplies for battery charging. In a resonant mode power supply, an oscillating circuit, which is typically a series oscillating circuit, is supplied through a first switching element with a direct voltage by which the oscillating circuit is excited charging a capacitor in the oscillating circuit. After one-half of an oscillating period, the first switching element is opened, and the oscillating circuit is connected to ground through a second switching element, and as a result, the capacitor of the oscillating circuit discharges, and the oscillation is thus continued. An oscillation can accordingly be excited by alternating opening and closing the two switching elements.
Present day television receivers are controlled by microprocessors. If power is completely removed from the television receiver when it is turned off, all of the parameters stored in the microprocessor, e.g., last volume level, last channel being watched, favorite channels, etc., are lost. In order to prevent this loss of information, the power supply includes a stand-by mode in which power is removed from the main operating circuits of the television receiver, but the microprocessor still receives sufficient power to retain the stored information, as well as to allow the microprocessor to reactivate the television receiver through the use of, for example, a remote control.
While, in a standard stand-by mode, a reduced amount of power is consumed, new specifications for television receivers require that the amount of power consumed during stand-by operation be lower than 5 watts. This is achieved in switched-mode power supplies by operating the power supply in a burst mode of operation, wherein the power supply operates at its normal frequency for limited periods of time (bursts) which are at a low frequency (e.g., 5-10 Hz).